Remnants of Ida produce historic flooding event for portions of the Northeast

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Radar Estimated Rainfall Totals for Northeast

ROCHESTER, N.Y. (WROC) — For millions of people across the mid-Atlantic and Northeast, Wednesday evening became a day they would remember for years to come. Subways, homes and apartment buildings were flooded with raging waters, and city streets became rivers as torrential rainfall and flooding wrecked havoc on some of the most populated areas of the country.

Nearly 48 hours after Ida made landfall on the Louisiana coast as a major hurricane, the remnants of Ida carved a path up the mid-Atlantic and produced record breaking amounts of rain, dangerous flooding and severe storms in places that don’t typically see events of this magnitude.

Not only was heavy rain and flooding an issue, but tornadoes that looked like they came straight out of tornado alley tore through neighborhoods and towns across places like Philadelphia and New Jersey. 

This is the kind of flooding that’s usually witnessed when a strong hurricane directly strikes an area, but the areas affected by just the remnants of what once was a major hurricane saw the unimaginable; life-threatening flash flooding in the most populated city in the U.S. So what happened to cause this?

Atmospheric Explanation

The reason why this flood event became so intense at the time and place it did, was because of a number of atmospheric factors. First off, NYC and surrounding areas had already received a ton of rain over the past month, so the already saturated soil likely played some role in how quickly water was able to flood where grass and soil were more prevalent. Another factor for cities especially like New York, was the fact that the water fell onto surfaces like concrete, asphalt and pavement that aren’t as permeable as grass and soil. This means the water was able to pool much quicker with no where for it to absorb or escape.

Second, what’s referred to in meteorology as a deformation zone occurred as Ida’s remnants merged with the jet stream current. Deformation zones occur within areas of opposing flow that are brought together. In this case, it was a region of high pressure creating northerly flow to the north of the incoming southerly flow from the area of low pressure that was Ida. This created an area in the atmosphere aloft where significant stretching and/or shearing could occur, creating an enhanced area of lift at the surface. The more lift you have, the heavier and more intense the rain/storms can be. This allowed all of the tropical moisture to be dumped out across the mid-Atlantic, and the results were astounding.

“1 in 100 Year Flood Event”

Many are calling this a “1 in 100 year flood event”, but what does that mean exactly?

“The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term “100-year storm” is used to define a rainfall event that statistically has this same 1-percent chance of occurring.” – USGS

For example, you can have multiple “1 in 100 year” floods occur within the same year. The title is a probability of an event occurring in the first place. It means there’s a 1 in 100 chance of the event happening every year, but it does NOT mean the event only happens once every 100 years.

The probability itself was low, but that doesn’t mean it was outside the realm of possibility. In this case, the almost impossible became possible. 

Many people are saying that this event “came out of nowhere,” when in fact it was forecasted and expected. I think the perception of what happened was so hard to grasp in the moment, because it’s hard to know and even understand what to expect outside the realm of your own human experience. This event was definitely a first for many people.

Although the event itself was definitely historic and unprecedented, it was not unexpected.

NYS Wednesday Rain totals

About 6-12” of rain fell with rainfall rates up to 4” per hour for many. Below are just some of the rainfall totals including upstate and western New York for perspective.

Staten Island 8.92″

Central Park 7.19″

LaGuardia Airport 6.89″

JFK Airport 2.77″

Albany 1.42″

Binghamton 0.67″

Rochester 0.0″

Syracuse 0.0″

Other Totals

Ringoes, NJ 9.91″

Coatesville, PA 9.43″

Waldwick, NJ 8.59″

For a more detailed summary with wind gusts and other rain totals from Hurricane Ida, click HERE.

~Meteorologist Christine Gregory 

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